1. Field of the Invention
This invention relates to a surface treatment method for increasing the adherence of plated metals to a base surface of polyimide. It also relates to a method for applying a metal coating to a surface-treated polyimide base material in an image-wise fashion.
2. Technical Background
Certain electronic assemblies involve the use of conductor traces on flexible dielectric substrate materials. Polyimides represent a preferred flexible dielectric substrate material, in part because of their excellent thermal stability and solvent resistance. Unfortunately, the direct adhesion of metals widely used in the electronics industry, for example gold, silver and copper, to polyimide base materials is quite poor without the use of adhesives. Further, the use of adhesives to form adhesive-bonded laminates is undesirable because, among other concerns, the resulting adhesive bonded laminates exhibit poor thermal and dimensional stability.
U.S. Pat. No. 4,517,254 discloses a method of metallization of a polyimide surface characterized in that the polyimide is pretreated with an aqueous solution of alkali hydroxide and an organic nitrogen compound such as N,N,N',N'-tetra-(2(hydroxypropyl)ethylenediamine, ethylenediaminetetraacetic acid and nitrilotriacetic acid. The utility of a number of other organic nitrogen compounds is disclosed, including some bearing sulfur atoms in the form of a sulfonic acid group. There is no disclosure nor suggestion of the utility of any thiol-containing compound for example, organic amino thiols, in such a process.
U.S. Pat. No. 4,775,449 discloses a method of improving the adhesion of metal applied on a polyimide surface characterized in that the surface is first treated with an adhesion promoting compound containing a nitrogen-oxygen moiety prior to plating of the metal thereon. The preferred adhesion promoting compound is a hydroxylamine. The preferred metal is copper. There is no disclosure nor suggestion of the utility of any thiol-containing compound for example, organic amino thiols, in such a process. There is no disclosure nor suggestion of the utility of any metal plating method besides electroless plating and electroplating.
U.S. Pat. No. 4,806,395 discloses a method for texturing the surface of a polyimide sheet by contacting the polyimide sheet with a one-phase solution of an amine, an alkali metal hydroxide, a water miscible alcohol and water. This textured coating can be coated with a layer of electroless nickel or cobalt and a subsequent layer of electrolytically applied copper to yield an adhesiveless laminate useful in the production of electronic circuitry. The polyimide surface is severely damaged by this process. No use of organic amino thiols is disclosed nor suggested.
U.S. Pat. No. 4,832,799 discloses a method for texturing the surface of a polyimide sheet by contacting the polyimide sheet with a one-phase solution of an alkylenediamine, an alkali metal hydroxide, a water miscible alcohol and water. This textured coating can be coated with a layer of electroless nickel or cobalt and a subsequent layer of electrolytically applied copper to yield an adhesiveless laminate useful in the production of electronic circuitry. No use of organic amino thiols is disclosed nor suggested.
U.S. Pat. No. 4,842,946 discloses a method for treating a polyimide surface in which an adhesion promoting compound contacts the surface prior to the electroless plating of metal thereon. The preferred adhesion promoting compound is thiourea. The preferred metal is copper. There is no disclosure nor suggestion of the utility of any thiol-containing compound for example, organic amino thiols, in such a process. There is no disclosure nor suggestion of the utility of any metal plating method besides electroless plating and electroplating.
A major disadvantage of previously described methods for increasing adhesion between polyimide surfaces and applied metallic layers is that the adhesion promoting treatments are overly aggressive and cause damage to the polyimide sub-surface layer. The process of the present invention is mild enough to cause no damage to the polyimide sub-surface. It can be used successfully on extremely thin films of polyimide material without damaging the thin film. In addition, unlike the previously described methods, surface treatments in the present invention is reduced to a single step.